The present invention relates to the field of optical transmission, and more particularly to the limitations caused by the Raman effect in optical fiber transmission systems. The invention applies particularly to wavelength division multiplex (WDM) transmission systems.
WDM has made it possible to increase the capacity of optical fiber transmission systems quite considerably. Nevertheless, the Raman effect, or more precisely the crosstalk due to stimulated Raman scattering (SRS) constitutes a major limit; this effect is described, for example, in the work by G. P. Agrawal, entitled “Nonlinear fiber optics”, published by Academic Press 1980, for a signal of bandwidth less than or equal to 13 terahertz (THz) (440 cm−1). The effect leads to energy being transferred between the channels. For a WDM transmission system, the Raman effect causes gain to be shifted or the spectrum to be tilted after transmission. In other words, a spectrum presenting a plurality of channels at substantially identical power at the beginning of propagation presents, after propagation and because of the Raman effect, lower power levels for those channels at shorter wavelengths. A known solution to this problem consists in adapting the gain of the amplifiers used. Nevertheless, the range of corrections possible with that solution is limited.
N. Zirngibl, in “Analytical model of Raman gain effects in massive wavelength division multiplexed transmission systems”, published in Electronics Letters, Vol. 34, No. 14 (1998), pp. 789-790, proposes a model of the effects of Raman crosstalk showing that the spectral distortion induced by stimulated Raman scattering crosstalk depends only on the total injected power, and not on the spectral distribution of that power. In that article, distortion is modelled by perfect “tilt”, and provision is made for compensation by a linear fiber presenting tilt that is constant in terms of decibels per nanometer (dB/nm).
By way of example, spectrum tilt is described by S. Bigo et al. in “Investigation of stimulated Raman scattering on WDM structures over various types of fiber infrastructures”, published as an OFC '99 paper WJ7, Feb. 21-27, 1999. That document measures the effects of Raman crosstalk in the wavelength range situated around 1550 nm, but it does not propose any solution to the problem.
D. N. Christodoulides and R. B. Jander in “Evolution of stimulated Raman crosstalk in wavelength division multiplexed systems”, published in IEEE Photonics Technology Letter, Vol. 8, No. 12, December 1996, pp. 1722-1724, proposes a digital simulation of the crosstalk caused by the Raman effect between the various channels of a WDM transmission system. That document uses a triangular approximation to the Raman gain profile over the multiplex.
A. R. Chraplyvy in “Optical power limits in multi-channel wavelength division multiplexed systems due to stimulated Raman scattering”, published in Electronics Letters, Vol. 20, No. 2 (1984), pp. 58-59 also proposes a triangular approximation for Raman gain in a WDM transmission system; it is specified that the models provided in that document can be used for estimating the limitations induced by crosstalk due to the stimulated Raman effect. That document does not propose a solution to the problems raised by such amplification.
A single coherent light wave emitted into a monomode fiber is subjected to losses associated with spontaneous generation of a second wave, and then to it being amplified, because of the Raman effect. The frequency of the resulting wave is reduced by about 13 THz relative to the initial wave. T. Sylvestre et al. in “Stimulated Raman suppression under dual-frequency pumping in single mode fibers”, published in Electronics Letters, Vol. 34, No. 14 (1998), pp. 1417-1418 describes an experimental setup enabling these loses to be greatly limited for the wave of interest by eliminating the resulting wave. To do this, a wave whose frequency is about 2×13 THz less than the frequency of the wave of interest is also emitted into the fiber, and a polarization-maintaining fiber is considered. That method of reducing the Raman effect is not applicable to a broad band or a very broad band transmission system.
The French patent application filed on Jun. 10, 1999 under the No. 99/07324 and entitled “Compensation de l'effet Raman par pompage dans un système de transmission à multiplexage en longeur d'onde” [Raman effect compensation by pumping in a wavelength division multiplexed transmission system], proposes injecting pumps at wavelengths lower than those of the signals of a wavelength division multiplex into a link in order to compensate for the tilt caused by the Raman effect on the channels of the multiplex. The energy provided by the pumps compensates for the losses from the initial channels of the multiplex. That solution is proposed for signals at wavelengths lying in the range 1520 nm to 1600 nm, i.e. over bandwidths that are generally less than 80 nm to 100 nm, and that are at most equal to 20 THz.
All of those documents of the state of the art, and also the last-mentioned French patent application, apply to a transmission band centered around 1550 nm, i.e. to wavelengths in the range 1520 nm to 1600 nm.
Kenneth L. Walker in “Status and challenge of optical amplifiers and lasers”, published at OAA '98, MB1, pp. 12-14, mentions as a future development the use of the entire spectrum width that is available in optical fibers, i.e. bandwidths of 400 nm or even more in the range 1.2 micrometers (μm) to 1.7 μm. That document specifies that the two main factors which limit the use of the entire spectrum available in a fiber are dispersion and attenuation. It further states that Raman amplification constitutes a good candidate for amplification over a broad band, unlike rare earth doped amplifiers which operate only over a bandwidth of less than 100 μm. To ensure gain flatness, that article proposes using long-period gratings as a filter; the examples proposed show filters in the range 1500 nm to 1600 nm and the resulting gain over a bandwidth of 40 nm.